Pyridinolyses of 2,4-Dinitrophenyl Phenyl Carbonate and 2,4-Dinitrophenyl Benzoate: Effect of Nonleaving Group on Reactivity and Mechanism￠ó

摘要

Second-order rate constants (k_N) have been measured for reactions of 2,4-dinitrophenyl phenyl carbonate (2) with a series of pyridines in 80 mol % H₂O/20 mol % DMSO at 25.0 【 0.1 ^oC, and compared with the k_N values reported for the corresponding reactions of 2,4-dinitrophenyl benzoate (1) to investigate the effect of nonleaving group on reactivity and mechanism. The reactions of 2 result in larger kN values than those of 1. The Br┆nsted-type plot for the reactions of 2 exhibits a downward curvature (i.e., モ₂ = 0.84 and モ₁ = 0.16), which is typical for reactions reported to proceed through a stepwise mechanism with a change in rate-determining step. The pK_a at the center of the Br┆nsted curvature, defined as pK_a^o, has been found to be 8.5 and 9.5 for the reactions of 2 and 1, respectively. Dissection of k_N into the microscopic rate constants (e.g., k₁ and k₂/k_-1 ratio) has revealed that the reactions of 2 result in larger k₁ values than those of 1, indicating that PhO behaves as a stronger electron-withdrawing group than Ph. However, the k₂/k_-1 ratio has been found to be independent of the electronic nature of Ph and PhO.